[ESSI-1336] Resolve bugs loading dynamic schema

[aploshay]

As a preparatory step for satisfying ESSI-1336 (bulkrax CSV import respecting a supplied dynamic_schema_id), resolves 2 allinson_flex issues:

• #dynamic_schema always returns the newest schema samvera-labs/allinson_flex#61
• #initialize overrides a supplied dynamic_schema_id attribute samvera-labs/allinson_flex#63

Ways to manually check these fixes:

• Issue 61 regards how calling work.dynamic_schema always returns the most recent schema. This can be checked in console against an earlier-saved work with an older schema, with work.dynamic_schema_id == work.dynamic_schema.id -- it should return false without the fix, true with it.
• Issue 63 regards how initializing a work with an older schema would still result in using the most recent schema. This can be checked in console, with work = WorkType.new(dynamic_schema_id: OLDER_ID_VALUE); work.dynamic_schema_id == work.dynamic_schema.id -- it should return false without the fix, true with it.